Memory Addressing & Segments

• Memory locations 00390H through 00393H contain 9A, 76, 65, and 1F respectively, assuming DS=0030H, SI=0090H, and BP=0002H.

Question 1 possible answers

• If AX is moved from [SI], AX contains 9AH.
• If AX is moved from [SI+1], AX contains 76H.
• If AX is moved from [SI][BP], AX contains 65H.
• MOV AX, [BX] is an example of register indirect addressing mode.
• In Real Mode, a memory segment that begins at 10000H ends at 1FFFFH.
• The physical address accessed by the segment:offset combination A359:B3FD is AE98D.

TI Bit

• If TI=0, the parameter is a global descriptor table.

Addressing Mode

• Indirect addressing mode uses a register to hold the actual address for data transfer.
• With CS containing 2001H and IP containing 007CH, the physical memory location accessed is 2008CH.

Common Physical Memory Address

• The following segment:offset combinations point to a common physical memory address: A1B2:000C, A1B0:200C, and A1B1:001C.
• Offset address selects any location within the 64K byte memory segment.
• Given CS = 2000H and IP = 1000H, the memory address of the next instruction is 21000H.

Real Mode

• In real mode, the physical address accessed by OCD1:02E0 is OCFF0H.
• The IP register contains the location where the next instruction in the code segment should be fetched.
• If a descriptor has Base = 23000000H, Limit = 012FFH, and G = 1, the described segment ends at 242FFFFFH.
• Real mode memory exists at locations 00000H-FFFFFH.
• MOV [SI], BH writes a byte of data from BH into memory location 1000H, given SI = 1000H.
• The selector selects the descriptor from a descriptor table.

Segment Registers

• The segment registers should only contain segment addresses.
• In protected mode, access to the segment is allowed if both RPL <= DPL and RPL > DPL.
• Protected mode memory protection is implemented by restricting access through privilege levels and access rights.
• MOV AX, 1234H is an example of immediate addressing mode.

Addressing Mode Types

• Direct Mode is when the effective address of the memory location is visibly part of the instruction.
• To form a physical memory address, segment register contents are shifted left by 4 positions and added to the 16-bit offset address.
• In protected mode, a selector value gives segment information including base address, access rights byte, and ending address.
• For 80286-Core processors, protected mode memory exists at locations 000000H-FFFFFFH.
• The first 1 MB of memory is called both conventional and real mode memory.
• MOV DS:[2000H], AL has mixed-size operands.
• The descriptor describes a memory segment beginning at AB208000H and ending at AC20AFFFH, which is a writable data segment, grows upward, 32bit, 2nd highest privilege and has not been accessed, the descriptor is: ABH D0H B2H 20H 80H 00H 01H 02H
• With DS=1A00H and ECX=00002000H, the memory location is 1C000H.
• With DS = 1000H, SS = 2000H, BP = 1000H, and DI = 0100H, with real mode operation, MOV AL, [BP+DI] accesses memory address 21100H.
• The instruction MOV AX, 2020H uses Immediate Addressing Mode and moves 16 bits of information.
• The descriptor describes size, boundaries, and access rights in the memory segment.

Memory Locations

• In stack memory-addressing modes, the low-order 8 bits are placed in the location addressed by the Stack Pointer.
• Immediate Addressing Mode is where you directly specify the operand value..
• An intersegment jump is a jump to any memory location within the entire memory system.